Stang70Fastback Posted May 3, 2024 Share Posted May 3, 2024 (edited) Hello folks! I'll try to get you up to speed as concisely as possible. Bought a '93 HPC last year. Went to drive it 800 miles home (very hot day.) Only made it about 200 miles before we noticed it was overheating. Trailered it home. Replaced the water pump. Still overheating. Replaced the thermostat. Same. Replaced the sender. Replaced the Gauge. Replaced the whole damned radiator and fan. Jacked the car way the hell up and burped all the air. I FINALLY seem to have found the issue. Turns out it wasn't overheating at all. Instead, when the radiator fan kicks on, the temperature gauge jumps up by like 30 degrees. That means on hot days (like say trying to drive 800 miles home on a 90 degree, sunny day, thrashing up twisty mountain roads with a girlfriend and a trunk full of luggage), the engine might tend to run a bit warm... and then you add 20-30 degrees to that reading, and suddenly you have a gauge showing close to pegged near 250. SO, now that I seem to know what the problem is... I need ideas on how to troubleshoot this. The only other gauge that seems to be affected is the oil pressure gauge, which drops when the fan kicks on. I assume that a bad ground is the issue, but I'm not sure what to look for. The grounding strap from the engine to the battery looks to be in good shape. This also happens even if the car is sitting in the garage with a freshly charged battery connected to a battery tender. Is there anything specific for me to look at that might help narrow down the issue? Any common things that tend to maybe cause this exact problem for people? Any help would be appreciated! I know the car isn't overheating now, but it's STILL unnerving seeing the gauge reading that high, and I'd like to fix it completely if possible! Edited May 3, 2024 by Stang70Fastback Link to comment Share on other sites More sharing options...
MV8 Posted May 3, 2024 Share Posted May 3, 2024 The instrument ground loom grounds at the wiper motor bolt so you might try removing, wire brush, reinstall. The cooling fan has a separate circuit from the heater blower/instruments/fuel pump circuit. May want to verify by pulling the fuses to see what works and what doesn't for each fuse. If it isn't easily corrected/determined, you might use the existing fan power to control an add-on relay to be located next to the fan and fed by the alt batt stud with 12-14 ga wire and an inline fuse holder. Then the amps through the green wire will drop from ten or so to less than one amp. The voltage at the fan will be higher as well so it will be more effective. 1 Link to comment Share on other sites More sharing options...
Stang70Fastback Posted May 3, 2024 Author Share Posted May 3, 2024 Ah! I didn't know about the ground at the wiper motor bolt. I will take a look at that and see if that's maybe my issue. The idea of running a separate circuit just for the fan isn't a bad idea either, but I'm hoping I can chase down a bad ground and that will mostly resolve it. Thank you! Link to comment Share on other sites More sharing options...
Stang70Fastback Posted May 3, 2024 Author Share Posted May 3, 2024 (edited) 4 hours ago, MV8 said: The instrument ground loom grounds at the wiper motor bolt so you might try removing, wire brush, reinstall. Is this what you are referring to? If so it does look clean, but I will still take it off and brush it. Edited May 3, 2024 by Stang70Fastback Typos Link to comment Share on other sites More sharing options...
MV8 Posted May 4, 2024 Share Posted May 4, 2024 Yes, that's it. There is room for improvement for a more reliable ground point. Paint is an insulator and this relies on the bolt shank contact with the drilled hole. For comparison, most oems use many small grounds with sheet metal type threads to provide the path to ground through the threads and a grease or sealer to prevent corrosion. I'd remove the bolt, sand, scrape or razor the paint off the bottom of the panel around the hole, add a finger wipe of grease to the surface, then fit a serrated washer under the head. Home improvement stores should have serrated washers that will fit well. Easiest to just take the bolt to the store to match it up with a washer size. I'm guessing 5/16-3/8" washer. A normal washer will work ok if the paint is fully removed where it contacts the panel. 1 Link to comment Share on other sites More sharing options...
Stang70Fastback Posted May 4, 2024 Author Share Posted May 4, 2024 (edited) 10 hours ago, MV8 said: Yes, that's it. There is room for improvement for a more reliable ground point. Paint is an insulator and this relies on the bolt shank contact with the drilled hole. For comparison, most oems use many small grounds with sheet metal type threads to provide the path to ground through the threads and a grease or sealer to prevent corrosion. I'd remove the bolt, sand, scrape or razor the paint off the bottom of the panel around the hole, add a finger wipe of grease to the surface, then fit a serrated washer under the head. Home improvement stores should have serrated washers that will fit well. Easiest to just take the bolt to the store to match it up with a washer size. I'm guessing 5/16-3/8" washer. A normal washer will work ok if the paint is fully removed where it contacts the panel. Sooo, I might have found my issue? Obviously everything is grounded to the chassis through that bolt, but then I was trying to identify where the chassis grounds back to the battery. I know this photo isn't the best, but all I have on the battery is one big ground that goes to the engine block. However, from there another cable goes up and... when I checked to see where it went, it just popped out from under the carb and was apparently not connected to anything. Where the hell is this supposed to go? I still find it odd there's no cable straight from the chassis to the battery, but maybe this just needs to be connected somewhere and all will be well. It isn't very long... I wonder if it's supposed to go to the starter which is right under there. There's no obvious ground from the starter actually. Just a tiny wire going up into the loom. Edited May 4, 2024 by Stang70Fastback Link to comment Share on other sites More sharing options...
MV8 Posted May 4, 2024 Share Posted May 4, 2024 Generally, a short ground strap across the left rubber engine mount and a strap from the battery negative to the bell housing provides the complete circuit. Without the chassis ground strap, then engine would still ground poorly through the throttle cable, clutch cable if not hydraulic, and the ujoint needle bearings to the axle through the parking brake cable. The starter grounds through the mounting bolts. 1 Link to comment Share on other sites More sharing options...
wdb Posted May 6, 2024 Share Posted May 6, 2024 I read the first post and immediately thought "engine is not grounding to the chassis". I once had a friend with an MGB that would melt throttle cables. The problem was the engine ground had broken, and the engine was using the throttle cable to complete the circuit. 1 Link to comment Share on other sites More sharing options...
Stang70Fastback Posted May 6, 2024 Author Share Posted May 6, 2024 Update: resolved! Took that loose cable and relocated it to use as an engine-to-chassis ground! https://www.racecargobrrr.com/post/overheating-issues-or-not 2 Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now